In hybrid integrated circuits used in communications and other types of systems, resistors formed in a thin film structure are frequently used. It is often necessary to adjust the resistance of the film resistors in the hybrid circuit after the circuit has been physically assembled. To increase the resistance of a film resistor, resistive film of the resistor is trimmed by forming a cut across an electrical current path in the resistor. Such cuts could be made in sections of the film to make the width of the path smaller and thereby increase the resistance. Also, the cuts could be made completely through a selected number of film "rungs" of a ladder structure. In either case, the cut may be formed by mechanical abrasion, chemical etching or laser vaporization of the resistive film material. In the mass production of hybrid circuits, the speed and accuracy of the trimming technique and the attendant control of such trimming becomes economically important. Since the advantages of laser trimming include (1) very high production rates, (2) greater flexibility in functional trimming and (3) tighter tolerances, laser trimming is the preferable technique.
Although the use of a laser in trimming film resistors improves the rate of production, the process still remains one of trimming small amounts and measuring a signal which is affected by the resistor being trimmed to determine and monitor the trimmed resistance value. Moreover, in cases where a plurality of resistors are formed on a single substrate, if one of the resistors is overtrimmed, the entire substrate has to be discarded, even though the other resistors may be within desired values or capable of adjustment thereto.
U.S. Pat. No. 3,699,649, which issued to D. A. McWilliams, discloses a method and apparatus for automatically trimming resistors in production quantities. A resistor substrate, which is to be trimmed, is positioned on a mechanical table. A set of probes which is coupled to a monitor sensing network, engages the substrate to monitor the resistance value during adjustment. A mask of a grid pattern based on the geometric requirements of the desired circuit of the film is positioned above the substrate. The mask has openings at required precision adjustment points. Fiber optic light pipes with appropriate focusing characteristics are joined to the points and focus light conducted by the fiber optics. The receiving ends of the fiber optic light pipes are then aligned at a support in a single line to receive a laser beam. The support at the receiving end of the light pipes is movable relative to the beam such that the receiving ends are rapidly placed in sequence within the path of the laser beam. The moving sequence may be coordinated with a programmed network which predetermines the final desired resistance value. The laser may be arranged to be responsive to the monitor circuit such that each resistor is individually and automatically trimmed to its final desired value.
The apparatus above monitors the resistance of the film resistor while it is being trimmed. It does not adjust the resistor as a functional part of a circuit in which it is used. Thus, other circuit characteristics which affect the operation of the circuit must be adjusted separately.
Consequently, there is a need for an apparatus which quickly trims a resistor as a functional part of a circuit in which it is used.